Electrochemical Quartz Crystal Microbalance Study of Borohydride

ARTICLE pubs.acs.org/JPCC

Electrochemical Quartz Crystal Microbalance Study of Borohydride Electro-Oxidation on Pt: The Effect of Borohydride Concentration and Thiourea Adsorption V. W. S. Lam,† D. C. W. Kannangara,† A. Alfantazi,‡ and E. L. Gyenge*,†,§ †

Department of Chemical and Biological Engineering, The University of British Columbia, Vancouver, BC, Canada, 2263 East Mall V6T 1Z3 ‡ Department of Materials Science and Engineering, The University of British Columbia, Vancouver, BC, Canada § Clean Energy Research Centre, The University of British Columbia, Vancouver, BC, Canada ABSTRACT: A systematic investigation of BH4- electrooxidation in 2 M NaOH on Pt was carried out using the electrochemical quartz crystal microbalance technique (EQCM). Four sets of experiments were conducted: (i) Pt in NaOH, (ii) BH4- (between 10 and 60 mM) on Pt, (iii) thiourea (TU) on Pt, and (iv) BH4- in the presence of TU (between 0.01 and 2.1 mM) on Pt. The BH4- electro-oxidation mechanism was developed by correlating the EQCM results with density functional theory (DFT) calculations from the literature revealing the energetically most favorable adsorbates. The surface coverage by key intermediates was estimated using the van der Waals molecular areas. On the anodic scan, the BH4- electrosorption was followed by dissociation of BH4,ad generating BHy,ad and (4 - y)Had, with y between 1 and 3, depending on the available surface sites. BHy,ad is further oxidized as a function of electrode potential in Eley-Rideal and Langmuir-Hinshelwood type mechanisms with the participation of OH- and OHad, respectively. The oxidative desorption on the cathodic scan at potentials between 0.1 and 0.5 V of strongly adsorbed intermediates such as BOHad and BH2OHad is essential for recovering the Pt electrocatalytic activity. TU adsorption on Pt produces a characteristic potential dependent adsorption-desorption hysteresis. Furthermore, at TU concentrations above 0.045 mM for 30 mM BH4- a bilayer is formed on the surface, which is stabilized by Lewis acid-base interactions between TU and BH4-. As a result, the BH4- oxidation overpotential is increased leading to incomplete oxidation, whereas the BH4- thermocatalytic hydrolysis is inhibited.

1. INTRODUCTION In the general context of direct liquid fuel cells, direct borohydride fuel cells (DBFCs) have received significant attention only over the past decade, targeting mainly portable electronic applications.1-3 One of the challenges for DBFCs is the electrocatalysis of the BH4- oxidation where instead of the complete 8 electron transfer a number of partial oxidation pathways (e.g., 4-6 electron exchanges) are possible generating H2 and BH4-y(OH)y- (with y between 1 and 3). A summary of the faradaic (i.e., electrochemical) and nonfaradaic (i.e., thermochemical) reaction pathways for BH4- oxidation has been provided.4 From a fundamental catalytic point of view, two approaches could be deployed to deal with H2 evolution, either to suppress it or to develop a catalytic system that would oxidize efficiently the evolved H2. The first one is referred to as the “direct” borohydride oxidation, while the second one is the “indirect” (via H2) oxidation route. A good example for the latter is the bimetallic PtRu system having a bifunctional role where Ru is acting as the main catalyst for thermochemical BH4- hydrolysis (with some r 2011 American Chemical Society

contribution from Pt as well) generating H2, which is oxidized in situ on the Pt sites.5 In the case of the direct borohydride oxidation, a major problem is that many electrode material candidates are also catalyzing the thermochemical borohydride hydrolysis. Gold was considered noncatalytic toward the hydrolysis reaction.6,7 Hence, Au was extensively investigated as a DBFC anode.8-10 Recent experiments aimed at identification of the surface adsorbed species by in situ Fourier transformed infrared spectroscopy (FTIR) and online electrochemical mass spectrometry (OLEMS) revealed in fact some degree of BH4- hydrolysis on Au at both open circuit and low anodic overpotentials, forming H2 and BH3OH-11,12a BH4 - þ H2 O f BH3 OH - þ H2

ð1Þ

Received: September 14, 2010 Revised: December 31, 2010 Published: January 20, 2011 2727

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The total number of electrons involved in the BH4 oxidation on Au as determined by the Koutecky-Levich analysis applied to rotating disk electrode (RDE) voltammetry data can reach the maximum of 8 electrons at high anodic overpotentials, while at low anodic overpotentials 4-5 electrons were identified.13,14 However, a more practical approach to the determination of the number of electrons contradicts apparently these findings. When borohydride-air cells were discharged at low current densities of 1 and 2.5 mA cm-2, thereby imposing low anodic overpotentials, the number of electrons calculated based on the discharge capacity was 7.5 and 7, respectively.15 The latter numbers agree very well with earlier measurements using the diffusion-controlled peak current density in static electrode voltammetry.6,16 The discrepancy among measurements is most likely due to the time scale of the experiments and the residence time of the BH4-y(OH)y- species (especially BH3OH-) near the electrode surface, which in turn is dependent on the electrode design and hydrodynamic conditions.12b,12c Measurements on a longer time scale and hydrodynamic conditions that favor the accumulation of the reaction intermediates in the vicinity of the electrode surface are increasing the probability for further oxidation, therefore generating a higher apparent number of electrons exchanged per BH4- molecule. Overall, the Coulombic (or faradaic) efficiency is determined synergistically by a large number of factors including the catalyst surface, electrode potential, BH4-/OH- concentration ratio, temperature, electrode design, hydrodynamic conditions, and time. The electro-oxidation of BH4- on Pt is more complex. The catalytic effect of Pt for the thermochemical hydrolysis reaction was discovered by H. C. Brown and C. A. Brown in 1962.17 More recently, H2 generators based on BH4- hydrolysis using various high-surface area supported Pt and Pt-alloy catalysts were proposed for fueling H2-O2 PEM fuel cells.18-21 The rate of thermochemical hydrolysis using 5%wt Pt/C is zero-order with respect to BH4- and first-order with respect to H2O, at BH4-/Pt molar ratios between 200 and 1500. The corresponding activation energy is 45 kJ mol-1.22 In the case of BH4- electro-oxidation on Pt, the number of electrons determined by the Koutecky-Levich analysis of RDE data was close to 8 (i.e., between 7.1 and 7.7) at low anodic overpotentials (i.e., in the underpotentially deposited hydrogen region of Pt) and decreased to 5-6 electrons at high overpotentials.13 Thus, the situation on Pt seems to be the opposite compared to Au. Employing the borohydride-air cell discharge experiment, 4 and 5 electrons were determined at current densities of 25 and 50 mA cm-2, respectively.15 To minimize the rate of thermocatalytic H2 evolution on Pt, the adsorption of thiourea (TU) was proposed by one of the authors.16 The hypothesis was that on the polycrystalline Pt surface different sites are active toward hydrolysis as compared to electro-oxidation. Therefore, if TU could selectively poison the catalytic sites responsible for hydrolysis the direct oxidation of BH4- would prevail on the Pt surface. The improved Coulombic (or faradaic) efficiency of BH4- oxidation on Pt in the presence of TU has been experimentally confirmed, but TU had also a negative effect of increasing the BH4- oxidation overpotential.15,23,24 This indicates that complete selectivity with respect to TU adsorption could not be achieved under the employed conditions, albeit a systematic investigation of the TU/BH4-/ OH- concentration ratios was lacking. The goal of the present study was to contribute to the understanding of the BH4- electro-oxidation on Pt by coupling cyclic

voltammetry with quartz crystal microbalance measurements to investigate in a systematic manner the BH4- concentration and TU adsorption effects.

2. EXPERIMENTAL METHODS 2.1. Cyclic Voltammetry. Cyclic voltammetry experiments were conducted with a 0.5 cm diameter (0.196 cm2) Pt disk electrode (Pine Inc.). The Pt disk was polished with a 1 μm diamond paste for 2 min and sonicated in 18 MΩ deionized water for 1 min. The electrochemical area of the Pt disk electrode was 0.276 cm2, determined by the hydrogen underpotential adsorption/desorption method in 0.5 M H2SO4 at 20 mV s-1. Thus, the surface roughness factor was 1.4. The current densities on the Pt disk are reported per electrochemical area. Prior to experimentation, the Pt disk electrode was subjected to electrochemical cleaning by carrying out 10 cycles between 0.9 V and þ0.6 VSHE in 2 M NaOH at 10 mV s-1. Note the cleaning step was performed in 2 M NaOH and not in acid electrolyte (e.g., 0.1-0.5 M H2SO4) as in other publications,13 to maintain fidelity with the actual experimental conditions. Furthermore, highly positive potentials, such as ∼1.4 VSHE employed in ref 13, were avoided during the cleaning step to minimize the irreversible restructuring and/or oxidative changes of the Pt surface. A typical three-electrode setup was used with Hg/HgO, 0.1 M KOH reference electrode (Materials Mates), and a Pt mesh counter electrode. All electrode potentials were converted to the SHE scale. Various concentrations of 150 mL of NaBH4-2 M NaOH solutions were prepared using 98%wt NaBH4 (certified ACS Fisher Scientific). NaOH (2 M) was used to ensure the longer term stability of NaBH4 and to match the commonly used electrolyte compositions in practical borohydride fuel cell applications. For all solution preparations, 18 MΩ deionized water was employed. Various thiourea (TU) concentrations in the working electrolyte were prepared from stock thiourea (Sigma Aldrich) solutions in 2 M NaOH. All solutions were purged with N2 (Praxair) for two minutes and then blanketed with N2 during experiments. The temperature was controlled at 298 K by a jacketed glass cell and a water bath. 2.2. Electrochemical Quartz Crystal Microbalance (EQCM) Experiments. Microbalance studies were carried out with 0.196 cm2 standard finished 9 MHz AT cut Pt resonators (Princeton Applied Research Inc.) sputtered on quartz. The quartz crystal microbalance (QCM) instrument QCA 922 (Seiko, Princeton Applied Research) was used in conjunction with a PARSTAT 2263 (Princeton Applied Research) potentiostat and the threeelectrode setup described above. All experiments were conducted using 25 mL of solution at 293 K. The Pt resonators were first dipped for 30 s in a solution composed of 1:1 volume ratio of 95%wt H2SO4 and 30%wt H2O2. Afterward, electrochemical cleaning was applied in 2 M NaOH identical to the procedure described in section 2.1. The electrochemical area of a representative Pt resonator electrode was 0.230 cm2 as determined by the underpotential hydrogen adsorption/desorption method in 0.5 M H2SO4 at 20 mV s-1, which corresponds to a surface roughness factor of 1.17. The QCM results were normalized with respect to the above electrochemical area for all experimental data sets. The QCM was operated simultaneously with cyclic voltammetry, which is referred to as EQCM. The differences in the resonant frequency were recorded as a function of potential, and the corresponding mass changes were calculated based on the 2728

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proportional relationship Δm ¼ -

Δf Cf

ð2Þ

where Δf = f(E) is the electrode potential dependent deviation from the fundamental resonant frequency of the piezoelectric material (Hz); Cf is a constant, referred to as the Sauerbrey constant; and Δm is the mass change per unit electrode area. Determination of the Sauerbrey constant and calibration of the EQCM were performed using the Cu underpotential deposition (Cu-upd) method. Cu-upd was carried out from a 2 mM CuSO4 and 0.1 M H2SO4 solution at 0.3 VSHE for 20 min. Afterward the deposited Cu was anodically removed. Figure 1a shows both the reference cyclic voltammetry scan of Pt in 0.1 M H2SO4 and the Cu stripping curve, while Figure 1b presents the QCM response generated by the anodic Cu stripping. After correcting the QCM measurement with the response from a blank scan without the Cu-upd layer, the frequency change was plotted versus the charge associated with the anodic Cu dissolution (Cu f Cu2þ þ 2e-), and the Sauerbrey constant was calculated from the slope of eq 3 (i.e., Faraday’s law applied to eq 2) ηΔQM ð3Þ Δf ¼ - Cf nF where F is the Faraday constant (96 484.5 C equiv-1); M is the atomic weight of Cu; n is the number of electrons transferred (n = 2); ΔQ is the charge per unit electrode area associated with the electrodissolution of Cu; and η is the corresponding current efficiency (assumed to be 1). The experimentally determined Sauerbrey constant for the 9 MHz Pt resonator was 168.2 Hz cm2 μg-1. The theoretical Sauerbrey constant for the 9 MHz Pt resonator in vacuum was reported between 179.225 and 183.36 Hz cm2 μg-1.26 The approximately 6-8% difference between the experimental and theoretical values is acceptable, and it can be caused by a number of factors such as the method used for experimental determination of Cf, the scan rate applied for the stripping voltammetry, surface roughness, and thickness of the deposited film. It was found that the experimental Sauerbrey constant decreases with decreasing film thickness, from 238.1 (for films thicker than 7 monolayers) to 166.7 Hz cm2 μg-1 (for monolayer).25 Thus, the experimental Cf determined in the present work matches fairly well the value reported in the literature for the Cu monolayer.

3. RESULTS AND DISCUSSION 3.1. EQCM of Pt in 2 M NaOH. First, the EQCM response of Pt in 2 M NaOH was recorded in the absence of BH4- to establish a proper baseline for comparison. The cyclic voltammogram in Figure 2a is characterized by the following three regions: (a) underpotentially deposited hydrogen (upd-H) adsorption/desorption (-0.70 to -0.50 V), (b) double layer (-0.50 to -0.35 V), and (c) the hydroxide-oxide region (-0.35 to þ0.60 V). It must be noted in alkaline solutions the adsorption of H2O does not influence the frequency response, hence no mass change due to H2O adsorption or desorption can be measured.27 Analyzing Figure 2a along the anodic scan direction, in the region of upd-H oxidation the surface mass remained virtually constant with respect to the beginning of the scan, instead of decreasing, as would have been expected due to the oxidative removal of adsorbed species. This indicates a low surface coverage by weakly adsorbed upd-H on Pt in 2 M NaOH.

Figure 1. Calibration of the EQCM by anodic stripping of the Cu-upd layer generated on Pt during 20 min exposure at 0.3 VSHE using 2 mM CuSO4-0.1 M H2SO4. (a) Pt cyclic voltammogram in 0.1 M H2SO4 and the Cu anodic dissolution wave. (b) Surface mass change per electrochemical Pt area during anodic Cu dissolution. Temperature: 293 K. Scan rate: 15 mV s-1.

It has been proposed in the literature that in alkaline media upd-H is engaged in a hydrogen-bonded network with H2O dipoles and OH- ions in the vicinity of the surface,28 which would explain the weakening of the Pt-H bond. As a result, QCM was unable to detect the adsorption-desorption process of upd-H in 2 M NaOH, similarly to H2O adsorption-desorption. This finding is also supported by the voltammograms extended into the overpotentially evolved hydrogen region (-0.9 V) when the oxidation waves for upd-H were undetectable (Figure 2b). During the longer scan duration in Figure 2b, the weak Had converted into H2,(g) by thermocatalytic recombination. Hence, the anodic response in this region was characterized only by the wave due to H2,(g) oxidation (Figure 2b). Moreover, in relation to Figure 2b, the overpotentially evolved H2 did not distort the QCM measurements due to the novel vertical cell design employed that mitigated the disengagement of gas bubbles. There were no signs of interference from gas bubbles such as noisy current and mass change signals and increase of the resonator mass in the bulk hydrogen evolution region due to adsorbed gas bubbles. As the potential was scanned anodically beyond the upd-H oxidation (Figure 2a), there was a gradual mass increase in the 2729



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Figure 2. EQCM response of the Pt electrode during cyclic voltammetry in 2 M NaOH. (a) negative potential limit: -0.75 V, 1st scan. (b) Negative potential limit: -0.9 V, 10th scan. Temperature: 293 K. Scan rate: 10 mV s-1. Scans started in the anodic direction.

double layer region due to adsorption of OH-. Starting at -0.05 V the resonator mass increased steeply, which was associated with the development of a broad oxidation wave between -0.05 and þ0.20 V (Figure 2). It is accepted in the literature that in this potential region OH- electrosorption (eq 4) takes place forming surface hydroxides.29 Two pathways can be envisaged for electrosorption: the first one involves adsorption with simultaneous charge transfer (eq 4) on “free” Pt sites (i.e., with only H2Oad), and the second pathway where the OH- previously adsorbed at the inner Helmholtz plane is oxidized (eq 5). xPt- ðH2 OÞy þ OH - f Ptx - OH þ yH2 O þ e Ptz - OH - f Ptz - OH þ e -

ð4Þ ð5Þ

Since the slope of the potential dependent mass increase in the entire hydroxide region was higher than in the double layer region

(Figure 2a), it is proposed that eq 4 is predominant because eq 5 would not have caused a different mass change compared to the double layer region. The exact stoichiometry of the Pt surface hydroxide cannot be easily established as discussed extensively in the case of acidic media by Angertstein-Kozlowska et al.30 However, looking at the cyclic voltammogram in Figure 2a, the two oxidation waves with peaks at 0.1 and 0.35 V, respectively, could imply two stoichiometrically different Pt-OH species as shown by eqs 4 and 5 formed possibly on two different surface sites (e.g., terrace vs defects). The only available literature for comparison is on Pt(111) in 0.1 M NaOH, in which case a second oxidation peak current of similar magnitude with the first one was recorded, with peak potential separation between 0.2 and 0.3 V depending on the state of the surface.31,32 The two oxidation waves were attributed to the formation of both weakly and strongly adsorbed OHad species.31 At the switching potential of 0.6 V on the first scan (Figure 2a), the mass further increased showing there were still available active 2730



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Figure 3. Surface mass changes of the Pt electrode during consecutive cyclic voltammetry scans in 2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1. Scans started in the anodic direction.

sites for reaction 4 to occur. On the reverse, cathodic scan (Figure 2a), the first reduction wave was very broad, between about þ0.10 and -0.35 V, coupled with a large mass loss. This reduction wave is in fact composed of two peaks close to each other, indicating two types of surface adsorbed species were involved in the reduction reaction such as Ptx-OH and Ptz-OH. Furthermore, some of the surface hydroxide species originally present on the surface at the start of the scan were also reduced because the mass loss in the cathodic direction exceeded the mass gain from the anodic scan. Successive potential cycling of the Pt resonator electrode in 2 M NaOH down to -0.9 V revealed that with each scan the mass gain on Pt during the anodic scan diminished (Figure 3). This indicates that the surface gets saturated with strongly adsorbed Pt-OH species which cannot be completely removed on the cathodic scan inhibiting, therefore, the readsorption of fresh OHon subsequent scans. The surface coverage of strongly adsorbed OH increases with cycling. After the first five cycles the surface coverage of OHad is high, and a significant anodic mass loss develops in the potential range immediately following OHad formation (between 0.2 and 0.6 V, Figure 3). This is likely due to reactions converting Pt-OH into species such as: Pt-O-, PtdO, or Pt2dO Pt- OH þ OH - f Pt- O - þ H2 O

ð6Þ

Pt- OH þ OH - f PtdO þ H2 O þ 1e -

ð7Þ

The rate of reaction 8 is strongly dependent on the OHad surface coverage, and it is catalyzed by OH-. The steep mass decrease on the anodic scan between 0.2 and 0.6 V for cycle number 11 (Figure 3) suggests the possibility of reaction 8 occurring preferentially after extended cycling as opposed to either reaction 6 or 7 that would have generated lower mass losses due to only one H atom eliminated per Pt-OH site. 3.2. EQCM Study of BH4- on Pt in 2 M NaOH. Figure 4 shows the BH4- cyclic voltammograms at a scan rate of 100 mV s-1

Figure 4. Effect of NaBH4 concentration in 2 M NaOH on the cyclic voltammograms on Pt. Temperature: 293 K. Scan rate: 100 mV s-1. Scans started in the anodic direction.

as a function of concentration. Four characteristic oxidation peaks are identified labeled as: a1, a2, c2, and c1. The most significant concentration effect was observed in the case of oxidation wave a1 where the peak potential shifted toward more positive values with increasing BH4- concentration coupled with an increase of the peak current density. Wave a1 can be broadly defined between -0.9 and -0.1 V, wave a2 between -0.05 and þ0.6 V, followed by the other two oxidation waves on the cathodic scan, c2 between þ0.6 and 0 V, and last c1 between -0.05 and -0.8 V. The mechanisms responsible for the four oxidation peaks are discussed based on the EQCM experiments with NaBH4 concentrations of 10 and 60 mM, respectively. Higher borohydride concentrations were selected to obtain insights under conditions that are more relevant for the fuel cell operation. Figures 5 and 6 show the EQCM results for 10 and 60 mM BH4-, respectively. The scan rate was 10 mV s-1 to ensure adequate synchronization between the electrode polarization and resonator frequency change measurements. On the anodic scan between -0.9 and -0.75 V (wave a1) (Figure 5), BH4- related adsorption accompanies the rising anodic current generating a mass increase of up to 3.5 105 ng m-2. This mass increase is much above the level which would have been expected based only on OH- adsorption (Figures 2a and b). Thus, a competitive adsorption takes place between BH4- related species and OH-, with the former prevailing. At potentials exceeding the peak a1 potential (E > Ep(a1) = -0.50 V), the electrode mass decreased due to the oxidation process (Figure 5). When the BH4- concentration was increased to 60 mM (Figure 6), the a1 peak potential shifted to a more positive value (-0.20 V), and the anodic peak current increased by a factor of about six. Similarly to Figure 5, at E > Ep(a1), the electrode mass decreased. The decrease in mass at potentials higher than the peak a1 potential for all the investigated BH4- concentrations suggests the consumption of the adsorbed reactant species that caused initially the mass increase and formation of reaction products that are easily desorbed from the surface. The processes 2731



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(EC) sequence of steps is proposed

Figure 5. Cyclic voltammetry and surface mass changes of the Pt electrode in 10 mM NaBH4-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1. Scan started in the anodic direction.

ð9Þ

BH4, ad f BHad þ 3Had

ð10Þ

The change in surface coverage associated with the rising portion of wave a1 was estimated with reference to the coverage at the start of the scan (at -0.9 V, Figures 5 and 6). Adsorption can occur already under open circuit conditions leading to hydrolysis according to eq 1; however, BH3OH- is not expected to accumulate on the surface, and DFT modeling showed it is weakly adsorbed.34 Using the calculation procedure presented in Appendix I, at 10 mM BH4- the surface coverage increase corresponding to the a1 mass gain (Figure 5) was equivalent to about 1.6 monolayers (ML) of BH4,ad. A similar calculation for 60 mM BH4concentration (Figure 6) revealed that the surface coverage increase with respect to the reference surface at -0.9 V was only slightly different, namely, 1.7 ML, in spite of about six times higher anodic current. Thus, it can be argued that in both cases the surface was saturated with BH4,ad near the peak a1 potential. As the potential was scanned more positive with respect to the peak a1 potential, BH4,ad was rapidly transformed into products that easily desorbed from the surface as shown by the Pt resonator mass decrease (Figures 5 and 6). Therefore, with respect to wave a1 three surface processes can occur: (a) H2,g evolution by thermocatalytic recombination of Had (Tafel step), (b) Had oxidation (Had þ OH- f H2O þ 1e-; referred to as the Volmer step), and (c) electrocatalytic oxidation of BHad. The relative rates of H2 evolution and Had oxidation depend synergistically on Had surface coverage, electrode potential, and temperature. The question is what is happening to the BHad? There are a number of theoretically feasible pathways.34 A useful experimental approach to complement the EQCM study is to determine the number of electrons exchanged by RDE. At 10 mM NaBH4 concentration peak a1 was associated with a virtually 6 electron transfer. Considering that a maximum of only 3 electrons could be generated by 3Had oxidation, it is proposed that wave a1 incorporates also the stepwise oxidation of BHad following the energetically favorable pathways revealed by DFT calculation, generating B(OH)3 which is weakly adsorbed on the surface.34 Thus

Figure 6. Cyclic voltammetry and surface mass changes of the Pt electrode in 60 mM NaBH4-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1. Scan started in the anodic direction.

responsible for the first oxidation wave a1 have been recently debated in the literature.13,15,16,33 Essentially, two schools of thought have emerged. One of them proposes an almost complete oxidation of BH4- (7-electron) between -0.6 and -0.05 V involving surface hydrides and forming oxidized species that poison the surface.13,15 The other explanation is based on catalytic hydrolysis of BH4- and oxidation of the evolved hydrogen.16,33 To interpret the oxidation wave a1 one must consider that BH4- chemisorption on Pt is thermodynamically favorable over the entire potential range including open-circuit.34 Density functional theory (DFT) modeling on Pt(111) showed the energetically most favorable pathway when there are sufficient free surface sites available is the dissociative adsorption generating BHad þ 3Had.34 Thus, for wave a1 the following electrochemical-chemical

BH4 - f BH4, ad þ e -

BHad þ OH - f BHOHad þ e -

ð11Þ

BHOHad þ OH - f BOHad þ H2 O þ e -

ð12Þ

BOHad þ OH - f BðOHÞ2, ad þ e -

ð13Þ

BðOHÞ2, ad þ OH - f BðOHÞ3, ad þ e -

ð14Þ

BðOHÞ3, ad f BðOHÞ3, aq

ð15Þ

Reactions 11-14 are electrochemical variants of the EleyRideal-type mechanism. Out of the total 8 electrons from reactions 9-15, including the theoretical 3 electrons from 3Had oxidation, in practice 6 electrons were determined, which indicates the contribution of the thermocatalytic H2,g evolution (2Had f H2,(g)). The thermocatalytic H2,g evolution and electrocatalytic Had oxidation in conjunction with the facile desorption of boric acid (eq 15) could explain the decrease of the Pt electrode mass at E > Ep(a1) (Figures 5 and 6). However, at the completion of wave a1 2732



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the Pt mass is somewhat higher than at the beginning of the scan indicating that some of the boron hydroxide intermediates formed in reactions 11-14 remain adsorbed. Continuing the scan in the anodic direction, between 0 and 0.6 V new oxidation waves develop (Figures 4-6). Depending on the scan rate and BH4- concentration, these waves can be more or less resolved into distinctive peaks. Higher borohydride concentration favored the peak behavior (compare Figures 5 and 6), referred to as peak a2 (Figure 4). The QCM measurements in the range of 0-0.6 V differed between the 10 and 60 mM BH4bulk concentrations (compare Figures 5 and 6). In the case of the higher BH4- concentration, the mass increased continuously with potential scanning, extending into the early part of the reverse cathodic scan, generating a mass gain of up to 9 105 ng m-2 (Figure 6). For the lower concentration, on the other hand, the mass of the Pt electrode resonator increased with potential only up to 0.2 V, reaching a value of 2 105 ng m-2, which was followed by a mass decrease and stabilization at a level almost identical to the start of the scan lasting up to the switching potential of 0.6 V (Figure 5). This potential region between 0 and 0.6 V corresponds to the electrosorption of OH- and formation of OHad on Pt, as discussed in section 3.1. For the hydroxide region of Pt in the absence of BH4-, the mass gain was as high as 1.1 106 ng m-2 on the very first scan at 0.6 V, but it dropped with subsequent scans stabilizing around 2 105 ng m-2 at 0.2 V (Figure 3, scan number 11). The latter mass gain can be considered representative for the intrinsic state of the Pt surface in the hydroxide region, and it corresponds to an additional OHad surface coverage increase of 0.5 ML with respect to the beginning of the scan (Appendix I). With BH4- present, due to coadsorption of a number of species such as OHad, and some oxidized species remaining from wave a1, the minimum 9 free adjacent Pt atoms necessary for the dissociative adsorption of BH4,ad according to eq 1034 are most likely not available. Instead, it is proposed that wave a2 involves another type of dissociative adsorption requiring only 1-3 free Pt atoms34 BH4 - f BH3, ad þ Had þ 1e ð16Þ Borane (BH3), if desorbed, is unstable in aqueous solutions and forms either BH3OH-aq or B2H6,g. On the surface, BH3,ad can readily react with OHad according to a Langmuir-Hinshelwood mechanism ð17Þ BH3, ad þ OHad f BH2 OHad þ Had followed by dissociation of BH2OHad providing there are sufficient surface sites available BH2 OHad f BOHad þ 2Had

ð18Þ

Once BOHad was generated, it follows the same oxidative pathway as described by eqs 13-15 forming B(OH)3,aq and H2,g. Thus, it is proposed that oxidation wave a2 is mainly composed of reactions 4 and 16-18, followed by 13-15. The total number of electrons transferred is 4, corresponding to the overall stoichiometry expressed by eqs 19 and 20. RDE experiments confirmed this hypothesis revealing a total number of electrons for wave a2 of 4.7 on average. The somewhat higher value indicates some additional contributions by high potential oxidation of adsorbed species formed in the previous wave a1. BH4 - þ 3OH - f BðOHÞ3 þ 2H2 þ 4e -

ð19Þ

BðOHÞ3 þ OH - SBO2 - þ 2H2 O

ð20Þ

For the 10 mM BH4- concentration at the switching potential of 0.6 V, the electrode mass returned to a level only slightly higher than at the start of the scan (Figure 5). This indicates near complete consumption of all the adsorbed species including OHad. In the case of high BH4- concentrations (e.g., 60 mM), there is a build up of adsorbed species up to and beyond 0.6 V (Figure 6). The availability of OHad could become rate limiting, and there might not be enough free surface sites for complete dissociation of BH2OHad according to eq 18 leading to the accumulation of the latter on the surface. Furthermore, if OHad is unavailable, BH3,ad could react with OH- from the solution based on and Eley-Rideal type of mechanism ð21Þ BH3, ad þ OH - f BH2 OHad - þ Had Hence, BH2OHad (or BH2OHad-) and BOHad accumulate on the surface and are responsible for the increased Pt mass over the entire region starting from 0.1 V up to 0.6 V (Figure 6). Therefore, these species will play an important role on the reverse cathodic scan. In addition to BH2OHad or BH2OHad- generated by reactions 17 and 21, another source could be the dissociative adsorption (or electrosorption) of BH3OH- formed in the pH-dependent thermocatalytic hydrolysis of BH4- (eq 1) BH3 OH - f BH2 OHad - þ Had

ð22Þ

BH3 OH - f BH2 OHad þ Had þ e -

ð23Þ

or In the literature, it was shown that the oxidation of BH3OHtakes place exactly in the potential domain of wave a2, and it is a 3-6 electron process.13 The lower the pH and higher the temperature the more significant the contribution of reaction 1 and BH3OH-. Subtracting from the total mass gain at 0.6 V (Figure 6), the estimated contribution of OHad from Figure 3 (scan number 11) leads to approximately a 0.6 ML coverage increase due to BH2OHad at 0.6 V in the case of 60 mM BH4- concentration. On the reverse cathodic scan again two oxidation waves, c2 and c1 can be identified, with peak potentials of 0.3 V (c2) and -0.05 to -0.15 V (c1), (Figures 5 and 6). Peak c1 potential was somewhat scan rate dependent, shifting toward a more positive value at a slower scan rate (compare Figure 4 with Figure 5). The cathodic oxidation waves have a “cleaning” effect on the electrode surface especially at high borohydride concentrations. This effect is nicely revealed by the influence of the positive switching potential in Figure 7. The more positive the switching potential at the end of the anodic scan, the more negative is Ep(a1) on the next anodic scan. When the switching potential was only þ0.1 V on the subsequent (2nd) scan, Ep(a1) = -0.25 V, while switching at þ0.5 V on the following (6th) scan Ep(a1) = -0.42 V (Figure 7). Thus, the surface cleaning at 0.5 V lowered the peak a1 potential by almost 0.2 V. The efficient oxidative removal of adsorbed species is paramount to create the number of free sites necessary for the dissociative adsorption according to eq 10 (e.g., nine per BH4,ad34). The EQCM data in Figures 5 and 6 also support the proposition that the cathodic oxidation waves are removing adsorbed species. Wave c2 is in the potential region where Pt surface hydroxides and possibly some oxides are present on the surface. The EQCM measurement showed that at a potential E ∼ Ep(c2) the resonator mass decreased (Figures 5 and 6). 2733



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BH4- the already existent surface species are consumed. Furthermore, in the absence of BH4- in the same potential window, the Pt mass decreased due to the reductive removal of OHad (Figure 2), but there was no net reductive current measured with BH4- (Figures 5 and 6). The following scenarios are proposed for wave c1 as a function of BH4- concentration. In the case of 10 mM BH4- at potentials just preceding c1, around 0 V, there are some free surface sites that became available after the oxidation wave c2. It is proposed that in the first step BH4- readsorbs on the surface and reacts with OHad, generating in the first step BH2OHad. This reaction is responsible for the mass increase just preceding peak c1 (Figure 5) BH4 - þ OHad f BH2 OHad þ H2 þ e -

ð24Þ

At E ∼ Ep(c1), the overall reaction is BH2 OHad þ 2OH - f BðOHÞ3 þ 3Had þ 2e -

Figure 7. Effect of the positive switching potential on the cyclic voltammetry behavior of BH4- on Pt. Electrolyte: 20 mM NaBH4-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1. Scans started in the anodic direction.

Separate RDE experiments revealed that wave c2 was independent of the rotation rate (results not shown here), which suggests surface reaction controlled peak. In the case of 10 mM BH4- concentration at the end of the anodic scan, there are very few adsorbed species left on the surface, hence the peak c2 current density was very small, about 7 A m-2 compared to 100 A m-2 in the case of 60 mM BH4- (Figures 5 and 6, respectively). One possible option is the removal of BOHad following the sequence of reactions 13-15. Alternatively, the Langmuir-Hinshelwood-type mechanism can be envisaged between BOHad and OHad. Continuing further on the cathodic scan, the sharp oxidation peak c1 between -0.05 and -0.15 V is a characteristic feature of borohydride cyclic voltammograms (Figures 5 and 6). Koper showed by theoretical modeling that generally sharp voltammetric peaks are caused by competitive adsorption between small and large adsorbates.35 The peak c1 current density is strongly dependent on bulk BH4- concentration and scan rate (compare Figures 4 and 5). Increase of the peak c1 current density is favored by slower scan rate and higher BH4- concentration. This indicates that c1 is generated by a slow oxidation reaction which requires time to develop. Furthermore, this peak is strongly dependent on the switching potential; namely, the more positive the switching potential the more negative is the c1 peak potential and the higher its current density (Figure 7). In the literature, peak c1 has been related to the oxidation of either BH3OH- or adsorbed BH2(OH)22-.13,16 From the point of view of QCM, wave c1 was associated with a large mass loss: -1.3 106 ng m-2 in the case of 60 mM BH4- (Figure 6) and -7.9 105 ng m-2 for 10 mM BH4- (Figure 5). However, there is a difference in the QCM behavior for the two concentrations. In the case of the lower concentration, the sharp mass loss was preceded by a build-up (i.e., mass gain) on the surface (Figure 5), while for the higher BH4- concentration the mass of the electrode decreased directly from the high level it was preceding peak c1 (Figure 6). This means for 10 mM BH4- the oxidation wave c1 involves also a freshly adsorbed species, while for 60 mM

ð25Þ

Reaction 25 is responsible for the mass decrease on the Pt electrode, and the sharpness of the peak is due to exchange on the same surface site of the large adsorbate BH2OHad with a small one Had (by analogy to the adsorption cases discussed by Koper35). RDE experiments revealed a total of 6 electrons for peak c2, which is indicated by eqs 24 and 25, including 3 electrons from further electro-oxidation of 3Had under the high anodic overpotential existent at Ep,c1. For 60 mM BH4- concentration, the main difference is that there is no newly formed BH2OHad. Hence, the already adsorbed species formed in the anodic scan are engaged in the oxidative desorption. The Pt resonator electrode mass decreased abruptly in the potential domain of peak c2 (Figure 6). The oxidative removal of BH2OHad is essential to regenerate the catalytic activity and create the surface sites necessary for specific adsorption of BH4-. The latter is reflected by the Pt mass increase in the cathodic direction immediately following wave c2, which is much more pronounced in case of high bulk BH4- concentration (compare Figure 6 with 5). 3.3. EQCM Study of Thiourea (TU) on Pt in 2 M NaOH. Figure 8 presents the EQCM behavior of 1.5 mM TU in 2 M NaOH. Between -0.9 and 0.09 V, the resonator mass was virtually constant indicating that in this region the adsorption of TU was independent of potential. It is interesting to compare Figure 8 with the Pt response in 2 M NaOH (Figure 2b). The hydrogen oxidation (either H2,g or upd-H) was completely inhibited by 1.5 mM TU, whereas the cathodic H2,g evolution was minimized. Oxidation of TU commenced at -0.2 V with two distinct irreversible peaks at 0.26 and 0.48 V, respectively (Figure 8). The oxidation is associated with a mass gain on the surface of almost 2.5 106 ng m-2, starting at 0.1 V. The first TU oxidation peak at 0.26 V is due to reaction 26, whereas at more positive potentials, with a peak at 0.48 V, dithiodiformamidinium hydroxide is generated (eq 27).36-38 SCðNH2 Þ2, ad þ OH - f ½SCðNH2 Þ2 þ ðOHÞ - þ e - ð26Þ ½SCðNH2 Þ2 þ ðOHÞ - þ SCðNH2 Þ2, ad þ OH f ðSCðNH2 Þ2 Þ2 ðOHÞ2, ad þ e -

ð27Þ

The fact that at the end of the voltammetric cycle the Pt mass recovered completely its original value without faradaic reduction 2734



Figure 8. Thiourea (TU) cyclic voltammogram and associated mass changes on the Pt electrode. Electrolyte: 1.5 mM TU-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1.

on the reverse cathodic scan hints toward electrode potential dependent adsorption-desorption hysteresis of TU and its oxidation products. In the cathodic direction, two different slopes for the mass decrease can be observed (Figure 8): (a) moderate mass loss between -0.4 V < E e 0.25 V and (b) abrupt mass loss between -0.9 V e E e -0.4 V. The moderate mass loss can be attributed to desorption of the weakly adsorbed (SC(NH2)2)2(OH)2,ad, which is soluble in aqueous solutions.36-38 An estimate shows this mass loss could correspond to the removal of 0.7 ML of (SC(NH2)2)2(OH)2,ad (Appendix I). Further on the cathodic scan, extensive desorption started at -0.4 V where the resonator mass decreased by about 2.15 106 ng m-2 (Figure 8). If this entire mass change is attributed to desorption of TU, it would correspond to the removal of 3 ML with respect to the situation at the beginning of the scan (Appendix I). The potential of zero charge of Pt (EPZC) in 2 M NaOH estimated with the Bockris-Argade-Gileadi equation39 is -0.4 VSHE. It is accepted in the literature that at negative potentials the preferred adsorption mode of the zwitterionic TU40 is with the sulfur oriented toward the surface.41 Therefore, at E < EPZC = -0.4 V, the TU adsorption weakens due to the electrostatic repulsion between the negatively charged surface and the negatively charged sulfur atom pointing toward the surface. The weakened adsorption is reflected by the mass loss measured with QCM. At positive potentials, on the other hand, the TU adsorption mode changes with the molecular plane now parallel to the surface.38 This arrangement is favorable for multilayer adsorption of TU and electro-oxidative dimerization (eq 27). Moreover, the interaction between adsorbed TU and the molecular environment in the vicinity of the electrode surface such as hydrogen bonding with H2O and OH- plays a significant role in determining the various phases for the adsorbed layer and the associated electrochemical response.42 3.4. BH4- on Pt in 2 M NaOH: The Effect of Thiourea. Figure 9 shows the effect of increasing TU concentrations on the BH4- cyclic voltammograms. Figure 10 shows the corresponding

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Figure 9. Effect of TU concentration on the cyclic voltammetry of BH4- on Pt. Electrolyte: 30 mM NaBH4-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1.

Figure 10. Effect of TU concentration on the Pt surface mass changes during BH4- cyclic voltammetry. 30 mM NaBH4-2 M NaOH. Temperature: 293 K. Scan rate: 10 mV s-1.

EQCM response in the presence of TU. The BH4- concentration was constant at 30 mM. Increasing the TU concentration from 0 to 0.1 mM shifted the first BH4- oxidation peak a1 by þ0.3 V. Further increase of the TU concentration had virtually no effect on the peak potential, which stabilized at -0.1 V, but the peak current density gradually decreased (Figure 9). Analyzing Figure 10, an interesting effect of the TU concentration is revealed in the region of wave a1 (between -0.9 and -0.1 V). At low TU concentration (i.e., 0.015 mM, Figure 10), the electrosorption of BH4- according to eq 9 is responsible for a mass gain of 2.5 105 ng m-2 (or 1 ML). However, with increase of 2735


The Journal of Physical Chemistry C TU concentration, the surface mass increased as well in the potential region of wave a1, reaching an approximately constant maximum value of 5 105 ng m-2 between 0.9 and 1.5 mM TU (Figure 10). The mass gain of 5 105 ng m-2 in terms of BH4,ad corresponds to 2 ML. This seemingly counterintuitive observation can be explained by the fact that at higher TU concentrations Lewis adduct type molecular associations between TU (weak Lewis acid) and BH4- (Lewis base) are possible, leading to a stable bilayer build-up on the surface. The BH4- electro-oxidation process is hampered in such a layer, leading to incomplete oxidation products that accumulated on the surface, as shown by the very slight electrode mass decrease in the case of 0.9 and 1.5 mM TU at 0 V (Figure 10). Thus, it is proposed that the inhibited electro-oxidation is related to the stabilization of BH4,ad- and BH4,ad in the TU intercalated bilayer, which prevents the complete dissociation according to eq 10. This argument was further validated by calculating the number of electrons exchanged using RDE experiments. It was found that increasing the TU concentration reduced the number of electrons involved in BH4- oxidation at peak a1 from 6 to 5, providing further evidence for the proposed mechanism. The formation of the TU intercalated bilayer can also offer an explanation for the inhibition of the thermocatalytic hydrolysis reaction (eq 1). Furthermore, TUad inhibited completely the Had oxidation as shown by Figure 8. This effect also contributed to the higher BH4- oxidation potentials and lower number of electrons encountered for wave a1, starting at very low concentration of TU such as 0.015 mM (Figure 9). From 0.1 V and extending to the switching potential of 0.6 V, the mass of the resonator electrode increased sharply with TU concentration (Figure 10). This is dominated by the TU oxidative adsorption (Figure 8) with contribution from borohydride oxidation wave a2. On the reverse cathodic scan, two features are evident: (a) the virtual absence of peak c2 (at 0.3 V) even in the presence of only 0.015 mM TU (Figure 9) and (b) the unusual dependence of peak c1 (at -0.1 to -0.15 V) current density on TU concentration (Figure 9). In the potential range of wave c2, oxidized TU derivatives are heavily adsorbed on the surface (Figure 8) inhibiting virtually completely the electrochemical reactions involving borohydride species described in section 3.2. Regarding wave c1 with peak potential of -0.1 to -0.15 V, since the surface adsorbed intermediates generated from BH4oxidation on the anodic scan such as BH2OHad were not removed at high anodic potentials due to inhibition by TU, the oxidation current c1 increases at low TU concentrations as these intermediates are being oxidized (Figure 9). This is reflected by the sharp decrease of the Pt mass between 0.3 and -0.2 V (Figure 10), which is more pronounced than in the case of TU alone (Figure 8). However, at TU concentrations close to and above 0.1 mM, the oxidation wave c1 is inhibited as well (Figure 9), and ultimately the removal of adsorbed borohydride species was no longer possible as evidenced by the higher final mass of the Pt electrode at the end of the cycle with 1.5 mM TU (Figure 10).

4. CONCLUSIONS A systematic EQCM investigation of BH4- electro-oxidation on Pt was carried out in 2 M NaOH. The BH4- concentration in the EQCM experiments was between 10 and 60 mM. Four sets of

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Table A.I. van der Waals Volumes and Cross-Sectional Areas for Selected Relevant Molecules and Molecular Fragments V^ vdW (Å3 molecule-1)

̂ (Å2 molecule-1) σ

BH4

28.2

11.2

OH BH2OH

14.3 37.4

7.1 13.5

57.0

17.9

142.7

33.0

species

SC(NH2)2 (SC(NH2)2)2(OH)2

experiments were conducted: (a) Pt in NaOH, (b) BH4- on Pt, (c) thiourea (TU) on Pt, and (d) BH4- and TU on Pt. The electro-oxidation mechanism of BH4- on Pt was discussed with respect to the four voltammetric oxidation waves a1, a2, c2, and c1 and their QCM responses. The oxidation mechanisms were developed by correlating the experimental results with predictions based on DFT calculations from the literature showing the energetically most favorable pathways.34 The surface coverage of key intermediates was calculated by estimating the respective van der Waals molecular areas (Appendix I). On the anodic scan, BH4- electrosorption was followed by dissociation of BH4,ad generating BHy,ad and (4 - y)Had, with y between 1 and 3, depending on the available surface sites. BHy,ad is engaged, as a function of electrode potential, in Eley-Rideal and Langmuir-Hinshelwood-type electro-oxidation mechanisms involving OH- and OHad, respectively. OHad is formed by electrosorption on the Pt surface between -0.05 and þ0.6 V. The first BH4- oxidation wave a1 occurs over a broad range of negative potentials (between -0.9 and -0.2 V). This wave is a composite of both Had and BHad oxidations generating H2,g and B(OH)3,aq. Thermocatalytic recombination of Had is also taking place, decreasing the number of electrons that could be obtained from 8 to 6 electrons. The oxidative removal of key intermediates such as BH2OHad and BOHad either at high constant anodic potential (0.5 or 0.6 V) or during potential scanning in the reverse cathodic direction starting from 0.5 or 0.6 V is essential for recovery and regeneration of the electrocatalytic activity with respect to BH4- oxidation. On the cathodic scan, peaks c1 (-0.05 to -0.15 V) and c2 (at 0.3 V) were attributed to the oxidation of BH2OHad and BOHad, respectively. The presence of TU produced interesting effects. It is proposed that at TU concentrations higher than about 0.045 mM (with reference to 30 mM BH4-) Lewis adducts are formed between the two species leading to the build-up and stabilization of a borohydride based bilayer with intercalated TU. This adsorbed layer increases the BH4- oxidation overpotential while decreasing the rate of the thermocatalytic hydrolysis of BH4-.

’ APPENDIX I: SURFACE COVERAGE ESTIMATION The surface coverage change with respect to the start of the anodic scan was estimated from Δm ^ 3 NA 3 ðA.IÞ Δθ ¼ σ MW where Δm is the mass change measured by QCM per unit electrochemical Pt area; MW is molecular weight of the adsorbed species; NA is Avogadro’s number; and σ̂ is the surface area occupied per molecule. The surface coverage change Δθ obtained from eq A.I is expressed as (cm2molecule cm-2Pt) and is referred to as the number of monolayers (ML). The surface area occupied per molecule 2736


The Journal of Physical Chemistry C was approximated with the cross-sectional area of the hypothetical spherical van der Waals volume of the molecule. Obviously, this approximation does not take into account the dynamic situation on the surface, various possible configurations, and intermolecular interactions. However, it can offer an indication and starting point for the quantitative characterization of adsorbates. ^ ¼ πR 2 ðA.IIÞ σ where the apparent molecular radius is expressed in terms of the van der Waals volume !1=3 3V^ vdW ðA.IIIÞ R ¼ 4π The van der Waals volumes of the molecules and molecular fragments of relevance in the present work were calculated with the equation developed by Zhao and co-workers for noncyclic molecular structures43 N 4π X V^ vdW ¼ r 3 - 5:92ðN - 1Þ ðA.IVÞ 3 j¼1 j In eq A.IV, V^ vdW is expressed in (Å3 molecule-1); N is the total number of atoms in the structure; and rj is the atomic van der Waals radius.44 Table A.I presents relevant values for V^ vdW and σ̂.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 1-604-822-3217. Fax: 1-604-822-6003.

’ ACKNOWLEDGMENT The financial support of this work by the Natural Sciences and Engineering Research Council (NSERC) of Canada is greatly appreciated. The authors thank the reviewers for the insightful comments and suggestions. ’ REFERENCES (1) Ponce de Leon, C.; Walsh, F. C.; Pletcher, D.; Browning, D. J.; Lakeman, J. B. J. Power Sources 2006, 155, 172. (2) Ma, J.; Choudhury, N. A.; Sahai, Y. Renewable Sustainable Energy Rev. 2010, 14, 183. (3) Wee, J.-H. J. Power Sources 2006, 161, 1. (4) Lam, V. W. S.; Gyenge, E. J. Electrochem. Soc. 2008, 155, B1155. (5) Lam, V. W. S.; Alfantazi, A.; Gyenge, E. J. Appl. Electrochem. 2009, 39, 1763. (6) Mirkin, M. V.; Yang, H.; Bard, A. J. J. Electrochem. Soc. 1992, 139, 2212. (7) Santos, D. M. F.; Sequeira, C. A. C. J. Electroanal. Chem. 2009, 627, 1. (8) Cheng, H.; Scott, K. J. Appl. Electrochem. 2006, 36, 1361. (9) Cheng, H.; Scott, K.; Lovell, K. Fuel Cells 2006, 5, 367. (10) Atwan, M. H.; Macdonald, C. L. B.; Northwood, D. O.; Gyenge, E. L. J. Power Sources 2006, 158, 36. (11) Molina Concha, B.; Chatenet, M.; Maillard, F.; Ticianelli, E. A.; Lima, F. H. B; de Lima, R. B. Phys. Chem. Chem. Phys. 2010, 12, 1. (12) (a) Chatenet, M.; Lima, F. H. B.; Ticianelli, E .A. J. Electrochem. Soc. 2010, 157, B697–B704. (b) Molina Concha, B.; Chatenet, M. Electrochim. Acta 2009, 54, 6130. (c) Freitas, K. S.; Molina Concha, B.; Ticianelli, E. A.; Chatenet, M. ECS Trans. 2010, 33, 1693.

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Electrochemical Quartz Crystal Microbalance Study of Borohydride

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